The Sea Fairies by L. Frank Baum
This is an adventure in the undersea world. Mayre, whose nickname is Trot, and Cap'n Bill are turned into mermaids through fairy magic and visit Aquareine, the mermaid queen. While on a tour of the marvels of the deep they are made prisoners of Zog the Terrible, a magician who is part man, beast, serpent, and fish. He is out to destroy them in a contest of magic.
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